This invention relates generally to an electrostatographic printer and copier, and more particularly concerns a cleaning apparatus.
Blade cleaning of photoreceptors (i.e. imaging surfaces or photoconductors) is basically a simple and economical concept that has reliability concerns when used in middle and high volume copier or printer machine applications due to apparent random failures. Such random failures justify the reluctance to include blade cleaners in higher volume machines without, or even with, some back-up cleaning element. Alternative cleaning devices, including magnetic, insulative and electrostatic brush assemblies are invariably installed as the primary cleaning element in higher volume machine applications. Use of devices exhibiting predictive or deterministic failure modes also facilitate identification and resolution of cleaning problems rising from other sources. Manifestations of deterministic cleaning failures include, but are not limited to, photoreceptor filming and cometing. (Cometing is where material, including toner particles, become impacted onto the photoreceptor and adhere with such force that they cannot be removed by the shearing or scraping action of the cleaning element.) Specific failures can also be, in part, material related (for example, involving the toner and the additives).
In contrast, random cleaning blade failures can arise because of inherent variations or flaws in the elastomer blade material. Such nonuniformities or defects in the blade material can cause or permit repeated stresses and strains occurring in the cleaning member during normal copy operation to locally and prematurely fatigue the contacting blade edge at the point of the flaw. An additional random failure mode for blade cleaners can arise from undesired and objectionable developed image related enhancements or reductions in the blade edge/photoreceptor friction. Such variations in contact friction can lead to unacceptably large tuck-under of a normal doctor blade edge. (A doctor blade edge is where the cleaning edge of the blade acts in a chiselling motion. This differs from a wiper blade edge, which, as its name implies, operates in a wiping fashion against the photoreceptor surface being cleaned.) A large enough tuck or break in the blade/photoreceptor seal can permit untransferred toner and other residual debris to pass or leak under the blade preventing the intended and essential cleaning action. Inhomogeneities in blade/photoreceptor contact can not only lead to a decreased cleaning efficiency according to the aforementioned process, but in severe cases can result in catastrophic system failure when, for example, the friction between the blade and photoreceptor becomes so great that the entire blade "flips over" or reverses so as to go from a doctor to a wiper-like position. However, in spite of these problems with the use of a blade cleaner, the copier/printer industry trend is toward applying some form of a photoreceptor blade cleaning system in the higher volume products.
Various strategies have been implemented or proposed to enhance the contact properties between the blade and photoreceptor. These include: agitation of the blade against the photoreceptor to prevent build-up of material along the contact seal; addition of redundant members, such as disturber brushes to loosen or collect debris which might otherwise stress the blade element; addition of lubricants to the toner, photoreceptor and/or blade; and roughening of the photoreceptor surface to reduce the blade/photoreceptor contact area, and thus, the blade friction.
The first two of these strategies increases the mechanical complexity and the cost of the cleaning assembly. The addition of lubricants, in the third strategy, increases complexity and introduces compatibility issues. And, the fourth strategy can also introduce compatibility problems depending on how surface roughening (i.e. roughening is where micron size asperities are engineered into the photoreceptor surface with the specific intent to reduce the contact friction between the cleaning device and the surface) is introduced (for example, particulate additives to the bulk of the transport layer can degrade electrical and/or mechanical properties). However, any such surface asperities can be worn away in a normal machine copy operation and thus, limit any cleaning benefit (i.e. the reduction in friction between the surface and the cleaning device achieved by the introduction of the asperities can be lost if normal copy operation is sufficiently aggressive to erode the asperities and smooth the photoreceptor surface). Surface roughening can also have direct adverse effects such as the introduction of sites against which toner can become lodged. Photoreceptor surface roughening can also inhibit cleaning by reducing friction in such a way as to allow the cleaning blade edge to pass over toner and other surface debris. This residual material passed over by the blade can later become pressed into the photoreceptor surface by other parts of the cleaning blade so as to serve as nucleation sites for comet growth.
One of the most common "predictable" or non-random blade cleaning failures is cometing on the photoreceptor. This type of failure is generally encountered and resolved during program development. As previously stated, in cometing, material, including toner particles, become impacted onto the photoreceptor and adhere with such force that they cannot be removed by the shearing or scraping action of the cleaning element. Additional debris, including untransferred toner, and developer and/or toner additives and their residue, can be impacted against the asperity. Repeated passes during the copier or printer process can lead to the build-up of elongated crusty deposits in front of the asperity which eventually print out as spots on the copy. These elongated deposits are called comets.
Once again various strategies have also been implemented or proposed to deal with this type of blade cleaning problem, many of which are redundant with those already mentioned. Additional approaches to the resolution of cometing problems include: elimination of the material which impacts, or builds up in the tail; include additives to the toner and/or developer which lubricate the contacting surfaces and/or scavenge the offending material; and development of a photoreceptor surface or surface coating which has an inherent resistance to toner impaction and/or cometing.
The prevailing opinion as to the origin of comets in blade systems is that localized tucks in the cleaning edge allow the toner particles or comet heads to be compressed into the photoreceptor. Thus, it is surmised that cometing and the more random type of blade cleaning failures may be related.
Blade edge tuck characterizes the tendency of the blade edge to curl or roll under in response to the dynamic friction forces established between the moving photoreceptor and loaded blade. It is intuitive that any mechanism or interaction which increases contact friction will increase blade edge tuck. Furthermore, the degree or magnitude of tuck can vary from point to point along the contacting length, consistent with the flexible nature of the blade material. Generally, edge tuck is not uniform when a blade rides against a smooth photoreceptor, but fluctuates locally along the full contact length in response to localized differences in dynamic friction.
It has been demonstrated, for example, that photoreceptor surface asperities of appropriate dimension can induce an elastomer blade edge to ride in a position of reduced and near uniform tuck. Such enhancement in contact uniformity is attributed to a continuous local reseating of the regions of excessive edge curl or tuck induced by the random distribution of asperities at the photoreceptor surface. Reseating is herein defined as the correction of a nonuniformly contacting cleaning blade edge to a position of greater uniform contact. A continuously temporal and spatial local reseating of the blade edge thus prohibits and/or inhibits the build-up of large friction enhancements which can increase the edge tuck to the point where cleaning failure occurs.
It has been hypothesized that photoreceptor surface asperities can produce localized stress/strain relationships at the contacting blade edge which tend to counter the distortion (tuck) forces generated by enhanced blade/photoreceptor friction. Loading of the blade against the photoreceptor causes the asperities to form local microscopic indentations in the blade edge in a direction away from vertical to the photoreceptor surface. Such selective compression of the blade edge counteracts the localized lateral stretching or excessive tucking of the edge. Photoreceptor surface asperities thus introduce local forces into the cleaning problem which tend to pull or draw adjacent excessively tucked blade edge regions back into the desired compliance.
Evidence supporting the aforementioned hypothesis was obtained from extensive print studies using a commercial xerographic copy machine operating with a seamed belt photoreceptor, a stand-along blade cleaner, and toner without special additives to inhibit photoreceptor cometing. A result of some significance was that printable comets first became noticeable on the last photoreceptor panel cleaned after the belt seam, or in a position after the blade was in the longest intimate continuous contact with the photoreceptor surface. Printable comets were never observed in prints produced from the panel immediately after the belt seam--this is the section of the photoreceptor first encountered by the blade after the blade has passed the seam. It is believed that the bump or displacement received by the blade when passing over the belt seam (i.e. this displacement is experimentally observable) dislodges accumulated toner debris from the cleaning blade edge and also allows the blade to reseat and ride in a less tucked position, hence, the absence of comets in the first photoreceptor panel after the seam.
A standard blade holder is disclosed in a U.S. Pat. No. 4,083,633, issued Apr. 11, 1978 to A. L. Shanly. The present invention represents a development in the above-cited technology, and accordingly this reference is incorporated by reference in the present specification.
The following disclosures may be relevant to various aspects of the present invention and may be briefly summarized as follows:
U.S. Pat. No. 4,989,047 to Jugle et al. discloses an apparatus for cleaning an electrophotographic printer imaging surface. The cleaning apparatus includes a primary cleaner device and a secondary cleaning member. The secondary cleaning apparatus consists of a blade holder pivotally connected to the housing that holds a cleaning blade in frictional contact with the imaging surface.
U.S. Pat. No. 4,640,608 to Higaya et al. discloses an apparatus for cleaning a photoconductive surface. The cleaning apparatus includes a blade holder that detachably holds a cleaning blade between two members that are fastened together.